Result of reproduce process:
Expected result and the reason why you expect:
Based on my (likely incorrect understanding of https://www.ietf.org/rfc/rfc2045.txt which [].pack('M') should be compliant with, all newlines should be formatted as CRLF and should only exist together, not separately. Relevant lines:

2.1. CRLF

The term CRLF, in this set of documents, refers to the sequence of
octets corresponding to the two US-ASCII characters CR (decimal value
13) and LF (decimal value 10) which, taken together, in this order,
denote a line break in RFC 822 mail.

2.7. 7bit Data

"7bit data" refers to data that is all represented as relatively
short lines with 998 octets or less between CRLF line separation
sequences [RFC-821]. No octets with decimal values greater than 127
are allowed and neither are NULs (octets with decimal value 0). CR
(decimal value 13) and LF (decimal value 10) octets only occur as
part of CRLF line separation sequences.

2.8. 8bit Data

"8bit data" refers to data that is all represented as relatively
short lines with 998 octets or less between CRLF line separation
sequences [RFC-821]), but octets with decimal values greater than 127
may be used. As with "7bit data" CR and LF octets only occur as part
of CRLF line separation sequences and no NULs are allowed.

2.10. Lines

"Lines" are defined as sequences of octets separated by a CRLF
sequences. This is consistent with both RFC 821 and RFC 822.
"Lines" only refers to a unit of data in a message, which may or may
not correspond to something that is actually displayed by a user
agent.

See also issue #14352. We have decided to reject this change. The behaviour is now documented.

To those who don't read Japanese let me summarize: Ruby's pack behabiour was from Perl's and Perl also don't emit CRLF. This is supposedly because in UNIX variants, emails are written in LF only and, to convert them to CRLF is done in sendmail(1). We don't want to break things here.

See also issue #14352. We have decided to reject this change. The behaviour is now documented.

To those who don't read Japanese let me summarize: Ruby's pack behabiour was from Perl's and Perl also don't emit CRLF. This is supposedly because in UNIX variants, emails are written in LF only and, to convert them to CRLF is done in sendmail(1). We don't want to break things here.